REJUVENESCENCE IN NATURE. 127 



be, when the cell exhibited no other difference of its 

 sides and parts beyond the contrast of contents and 

 membrane, therefore was completely uniform over its 

 whole periphery. According to the description Nageli 

 gives of his family Protococcacese, Protococcus* may be 

 regarded as the representative of this stage, being a 

 genus of which the individuals are globular cells, pro- 

 ducing free, likewise globular germ-cells, in their contents, 

 after their vegetative growth is completed. But it 

 appears to me doubtful whether, strictly speaking, such a 

 condition of the cell, completely equal in its relations to 

 all sides, ever occurs. If Protococcus^ as is probable, 

 possesses moving (swarming) germ-cells, these cells will 

 doubtless be found to exhibit, in the moving stage, an 

 elongation, so as to have a principal axis, and two 

 different extremities, one of which bears the cilia, while 

 the coloured contents of the cell are crowded into the 

 other. In Hydrodictyon, which genus must likewise be 

 included in the family of Protococcacese, the cells, con- 

 nected into a net-like colony, have also a distinct longi- 

 tudinal extension in their ulterior development, without, 

 however, any difference in the two ends, since the 

 extremities are both joined in a similar manner to the ends 

 of the adjacent sister-individuals to form the network. 

 But a distinct contrast between the upper and lower end 

 of the cell presents itself in the same family, in the genus 

 Ascidium, the free cells of which attach themselves to 

 foreign objects by one end (that elongated and bearing 

 the cilia at the period of swarming), whilst the free 

 extremity (originally the more obtuse) becomes prolonged 

 into a short point. In the tubular cavity of the cell, tlie 

 contents form a coating over the inside of the wall, by a 

 simultaneous division of which, after the vegetative 

 developments completed, numerous germ-cells are formed, 

 finally swarming out of the expanding mother tube, 



* According to Nageli's limitation of this chaotic genus. (Vide Nageli, 

 ' Die neueren Algensysteme,' p. 153.) 



